Specimen alignment or prealignment systems precisely position and orient articles in automated manufacturing or processing facilities. For example, an alignment system can be used to align semiconductor wafers for testing or processing during the manufacture of integrated circuits. One such wafer alignment system is described in U.S. Pat. No. 4,457,664 of Judell et al.
The Judell et al. alignment station rotates the wafer about a rotational axis that is perpendicular to the plane of the wafer to scan its generally circular perimeter with a capacitive edge sensor. The station translates the wafer along only one lateral axis to keep the perimeter aligned with the capacitive edge sensor as the wafer is rotated. Based upon information provided by a scan of the perimeter, a microprocessor computes a correction vector representing the displacement between the alignment station rotational axis and the centroid of the wafer.
The alignment station repositions the wafer in accordance with the correction vector to center the wafer centroid on the rotational axis. Specifically, the wafer is rotated about the rotational axis and moved along the one axis of lateral motion until the centroid is approximately aligned with a predetermined centered position of the rotational axis. As a consequence, the rotational axis is temporarily offset from its centered position in accordance with the correction vector.
The alignment station disengages the wafer, returns the rotational axis to its centered position, and then re-engages the wafer. Since the wafer centroid is only approximately centered on the alignment station, the wafer edge is again scanned and the wafer repositioned until its centroid is accurately centered on the rotational axis. The centered wafer is then rotated into a selected orientation and transferred to a wafer characterization station for testing.
A disadvantage of alignment stations of this type is that the wafer must be centered on the axis of rotation preparatory to positioning and orienting the wafer for testing or processing. The reason is that the alignment station uses rotation to both position the wafer centroid and to orient the wafer prior to being transferred. The wafer must be centered, therefore, so that the rotation into the selected orientation does not also change the wafer centroid position.
Another disadvantage is that centering the wafer on the alignment station is a time-consuming process that decreases wafer throughput.
A further disadvantage is that the increased handling of the wafer required to center it can increase wafer contamination, and thereby decrease the yield of integrated circuits formed therefrom.